Interpretive Summary: Dairy cows obtain their protein needs partly from synthesis by the microbes living in the rumen, the first compartment of the cow's stomach. The rest of their protein needs come from dietary protein that escapes breakdown by these same microbes. This process may reduce the value of feeds because the microbes often degrade more protein than they re-synthesize, making it necessary to feed more protein which can lead to excessive nitrogen excretion in manure. It is necessary for farmers to know the extent to which feed proteins are degraded in the rumen so they can formulate diets for their cows that are both economical and environmentally friendly. Also, having standard feed proteins for which rumen degradation has been determined in cows would be very useful for developing and validating laboratory tests for measuring protein degradation. Equal amounts of protein from four typical feed supplements were fed to ten lactating Holstein cows that had rumen cannulas (holes put into their rumens by a veterinarian using surgery). Samples of digesta flowing out of the rumen were collected through the cannulas to measure the amounts of microbial protein synthesized and feed protein escaping breakdown. This research determined the proportions of the protein from four important feeds that escaped the rumen undegraded: 27% for solvent soybean meal (the most common protein supplement fed to dairy cows), 45% for expeller soybean meal, 60% for blood meal, and 73% for corn gluten meal. These percentages can be used by dairy farmers to compute the protein value of each supplement for milk production. These feeds also will be used as standards for calibrating and validating laboratory tests for measuring rumen degradation of proteins, allowing scientists to develop laboratory assays for evaluating the protein value of large numbers of different feeds.

Technical Abstract:
Ten ruminally cannulated lactating Holstein cows, that were part of a larger 5 x 5 Latin square trial studying the effects of feeding different proteins on milk production, were used to quantify flows of microbial and rumen-undegraded protein (RUP) in omasal digesta. Cows were fed total mixed rations containing (dry matter basis) 44% corn silage, 22% alfalfa silage, 2% urea, and 31% concentrate. The basal diet contained 31% high moisture corn; equal N from one of four protein supplements was added to the other diets at the expense of corn: 9% solvent soybean meal (SSBM); 10% expeller soybean meal (ESBM); 5.5% blood meal (BM); and 7% corn gluten meal (CGM). Omasal sampling was used to quantify total AA N (TAAN) and nonammonia N (NAN) flows from the rumen. Estimates of RUP were made from differences between total and microbial N flows, including a correction for RUP in the basal diet. Modifying a spectrophotometric assay improved total purine recovery from isolated bacteria and omasal samples and gave estimates of microbial TAAN and NAN flows that were similar to a standard HPLC method. Linear programming, based on AA patterns of the diet and isolated omasal bacteria and ruminal protozoa, appeared to overestimate microbial TAAN and NAN flows compared to the purine assays. Yields of microbial TAAN and NAN determined using any method was not affected by diet and averaged 32 to 35 g NAN/kg of organic matter truly digested in the rumen. National Research Council (NRC) equations underpredicted microbial N flows by 23 to 293 g/d (versus HPLC), 46 to 355 g/d (versus spectrophotometry), and 109 to 378 g/d (versus linear programming). Estimates of RUP (means from the HPLC and spectrophotometric methods) were: SSBM, 27%; ESBM, 45%; BM, 60%; and CGM, 73%. Except for CGM, RUP values averaged about 20 percentage units lower than the ruminal degradations reported by the NRC.